Two-component developers used in electrophotography are generally the mixture of a toner and a carrier. The carrier is used to give an optimum amount of properly polarized triboelectricity to the toner.
A resin-coated carrier comprising a core particle and provided thereon a resin layer is preferably used to improve the durability and triboelectrification of the carrier.
A spray coating method has conventionally been used for the formation of the resin coat layer. This method, however, has the problem that it is liable to cause the flocculation of the carrier particles to make them larger, which results in lower yield of the carrier having a prescribed size distribution and a longer production time thereof.
In order to solve the above problems involved in the spray coating method, there have been proposed the following techniques for the resin coat layer formation as disclosed in:
(1) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP O.P.I.) No. 235959/1988, in which core particles are coated in a dry condition with resin particles having a size of not more than 1/10 that of the core particles; PA1 (2) JP O.P.I. No. 35735/1979, in which core particles is coated with resin particles in a dry condition at a higher temperature than the melting point of the resin; PA1 (3) JP O.P.I. No. 118047/80, in which metallic core particles having the surface area of about 200 to 1300 cm.sup.2 /g are heated at 160.degree. to 343.3.degree. C. for 20 to 120 minutes in the presence of about 0.05 to 3.0% by weight of resin particles having a size of about 0.1 to 30 .mu.m; PA1 (4) JP O.P.I. No. 27858/1988, in which the surfaces of core particles are coated in a dry condition with resin particles having a size of not more than 1 .mu.m; and. PA1 (5) JP O.P.I. No. 37360/1988, in which a fine polymer particle layer is formed and sticked on the surface of core particles.
The above techniques disclose basically that core particles and resin coating particles are mixed to allow the resin particles to stick electrostatically on the core particles by means of a triboelectricity and that mechanical impact force and/or heat are then applied to the mixture to fix the resin particles on the core particles to thereby form a resin coat layer. The sticking condition of the resin particles on the surfaces of the core particles depends substantially upon a layer forming process in which a mechanical impact force and/or heat are applied.
In the above techniques (2), (3), (4) and (5), it is difficult to obtain resin-coated carriers having a prescribed particle size distribution at a high yield because in the layer forming process the resin particles electrostatically sticked on the core particles are melted at a high temperature for fixing, which in turn is liable to cause the melted resin particles themselves to stick or the core particles to stick each other via the melted resin particles. Further, there is involved therein the problem that the resin-coated carrier surface is liable to become uneven to cause unstable triboelectrification.
The above technique (1) makes it possible to obtain resin-coated carriers having a prescribed particle size distribution at a high yield because the resin particles electrostatically sticked on the core particles are fixed mainly by means of a mechanical impact force, while it involves the problem that a coating efficiency is low and a longer production time is required.
Under such circumstances, the present inventors have proposed the technique in Japanese Patent Application No. 239180/1988, in which magnetic particles having the weight-average particle size of 10 to 200 .mu.m and resin particles having the weight-average particle size of less than 1/200 of that of the magnetic particles are mixed uniformly in a mixing pot at 50.degree. to 110.degree. C. and an impact force is applied repeatedly to the mixture to thereby coat the magnetic particles with the resin. This technique, however, still has room for improving a coating efficiency and production time. That is, in the mixing process in which resin particles are electrostatically sticked on the core particles, the resin particles differ in the sticking amount and condition by an electrification of the resin particles, and the sticking density thereof becomes low due to the electrostatic repulsion between the resin particles. That makes it easy for the coating resin particles to transfer between the core particles and requires longer time to form a uniform resin coat layer in the layer-forming process in which the coating resin particles are fixed on the core particles.
Thus, the coating resin particles are mixed with the core particles in such a manner that they are sticked densely on the core particles and then a mechanical impact force is applied to the mixture while heating if necessary to minutely dispose the resin particles on the core particles and fix them, whereby a resin coat layer is formed. In performing simultaneously disposition and fixation of the resin particles, it is liable to take a long time to form an even resin coat layer. Further, there is involved the difficulty that the resin particles sticking on the core particles drop therefrom to lower the resin coating efficiency while a mechanical impact force is applied. The dropped resin particles are liable to stick each other to form larger particles, which make it difficult for themselves to stick on the core particles because of their poor ductility. In addition, the dropped resin particles are liable to adversely affect the electrophotographic characteristics of copied images.
Under such circumstances, there have been proposed the technique in which the surfaces of the core particles are treated to control the electrostatic friction therebetween to thereby increase the electrostatic cohesion of the core particles and the resin particles, as disclosed in Japanese Patent Applications No. 314158/1988 and 314159/1988; and the technique in which the addition of coating resin particles is controlled to improve the electrostatic friction between the core and resin particles, as disclosed in Japanese Patent Applications No. 79306/1989 and 79312/1989. These techniques, however, still require a longer production time and can not necessarily provide satisfactory resin-coated carriers.